Numerical and experimental study of laboratory and full-scale prototypes of the novel solar multi-surface air collector with double-receiver tubes integrated into a greenhouse heating system

The application of greenhouse technologies is rapidly expanding in China and other regions of the world with the continental climate. Such technologies are characterized by a considerable consumption of fossil fuels, especially in winter periods. Various renewable energy technologies are used to reduce the energy demand in greenhouses, including solar heating. Simpler designs of water or air solar collectors are preferred without sun-tracking devices and reduced dimensions and weight. In this work, a novel solar multi-surface air collector with double-receiver tubes, which does not require a sun-tracking device is proposed and has increased concentration ratio and sun rays convergence and reduced thermal losses. The laboratory and full-scale prototypes of the proposed novel collector were manufactured and tested in laboratory conditions during the December-January period as a part of the greenhouse with the active-passive ventilation wall and latent thermal storage, located in the Beijing Region. Results of experiments demonstrate that the solar collector increases the average temperatures of the greenhouse north wall inner surface by 7.5 °C, indoor air during the night-time by 1.8 °C and indoor soil by 1.5 °C. The novel collector provided active heat gains of 11.831 GJ during the December-January period and satisfied from 11% to 81% of the heat load. A lumped parameter mathematical model of the proposed collector was developed, and the comparison of simulation results and experimental data demonstrate its high accuracy in prediction of the collector performance. The model was used to study the influences of outdoor meteorological parameters, air velocity in the receiver tubes and their length on the performance of the collector. This data was used to determine the rational design parameters and air velocity of the full-scale collector. The model was also used to rationalize the variation of air velocity in the receiver tube during the day.